Coital rates, sex-selective infanticide, and sex ratios at birth.

Abstract

The author of this short article comments on the suggestion by Brewis and Underwood, in their report on Micronesia, that the high sex ratios at birth were due to the high coital rates (greater than 108-110). First, in Australia, the reported sex ratio of births conceived in the first month of marriage from 1908 to 1967 was 109. Mean reported marital coital rates during the first month of marriage for the United States and England are estimated to be 20 per month. The average coital rate is higher among women who conceive. If the variation of sex ratios across societies were explained by coital rates alone, Micronesian coital rates would have to be much greater than 20 per month throughout couples' reproductive lives, and 3 times the averages in postindustrial countries. Such coital rates have not been substantiated; other reasons may account for the high sex ratios in Micronesia. Brewis and Underwood stated that they did not believe sex-selective infanticide was an explanation of the high sex ratios; this may have been based on current practices. Yet past practices of sex-selective infanticide could influence current sex ratios, given that there is mathematical evidence that genetics may influence the probability of producing a child of a particular sex. Therefore, the average current sex ratio at conception and birth of a population depends on: the magnitude of the genetic component of the Lexis variance (the reproductive-lifetime probability of a couple's producing a boy at each birth); and 2) the extent of the sex-selective infanticide in the past. The total extent of the Lexis variance, including its genetic component, is unknown.